Statistics Ch 7

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Statistics Ch 7
2012-02-25 12:13:14

Producing Data: Sampling
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  1. Observational study
    • Record data on individuals without attempting to influence the
    • responses. We typically cannot prove anything this way.
  2. Experimental study
    • Deliberately impose a treatment on individuals and record their
    • responses. Influential factors can be controlled.
    • Observational studies of the effect of one variable on another often fail because the
    • explanatory variable is confounded with lurking variables.
    • Well-designed experiments take steps to defeat confounding.
  3. Population
    The entire group of individuals in which we are interested but can’t usually assess directly.
  4. Parameter
    a number describing a characteristic of the population.
  5. Sample
    • The part of the population we actually examine and for which we do have data
    • How well the sample represents the population depends on the sample design.
    • Statistic is a number describing a characteristic of a sample
  6. Bad sampling methods
    Convenience sampling & bias
  7. Convenience sampling
    Just ask whoever is around.
  8. Bias
    Opinions limited to individuals present
  9. Voluntary Response Sampling
    Individuals choose to be involved
  10. Bias
    Sample design systematically favors a particular outcome.
  11. Good sampling methods
    Probability or random sampling
  12. Probability or random sampling
    • Individuals are randomly selected.
    • Sampling randomly gets rid of bias.
  13. Simple random sample
    • (SRS) is made of randomly selected individuals. Each
    • individual in the population has the same probability of being in the sample. All possible
    • samples of size n have the same chance of being drawn.
  14. SRS
    Simple random sample
  15. How to choose an SRS of size n from a population of size N:
    • 1. Label
    • 2. Table B
    • 3.Stratified random sample
  16. Label
    Give each member of the population a numerical label of the same length.
  17. Table B
    Read from Table B successive groups of digits of the length you used as labels. Your sample contains the individuals whose labels you find in the table.
  18. Stratified random sample
    • a series of SRS performed on subgroups of a given
    • population. The subgroups are chosen to contain all the individuals with a certain characteristic.
    • The SRS taken within each group in a stratified random sample need not be of the same
    • size.
  19. Caution about sampling surveys
    • Nonresponse
    • Response bias
    • Wording effects
    • Undercoverage
  20. Learning about populations from samples
    • The techniques of inferential statistics allow us to draw inferences or conclusions about a
    • population from a sample.
    • Your estimate of the population is only as good as your sampling design Work hard to eliminate biases.
    • Your sample is only an estimate—and if you randomly sampled again, you would probably get a somewhat different result.
    • The bigger the sample the better.